Heat treating process



Feb. 11, 1941. D. A. HOLT HEAT TREATING PROCESS Filed Feb. 24, 1958 IN VEN TOR. DONALD A. HOLT M ATTORNEY.

Patented Feb. 11, 1941 UNITED STA ES PATENT oFricE HEAT TREATING PROCESSApplication February 24, 1938, Serial No. 192,396

14 Claims.

This invention relates to the heat treatment of metal articles and moreparticularly to heattreatment by means of fused salt baths.

Heat treating'operations for articles of various metals are oftencarried out by immersing the metal articles to be treated in a fusedsalt bath. This method. of heat treatment is advantageous because of therapid transfer of heat from-the salt bath to the metal article's, thusproviding a rapid process well adapted to large scale production.However, difficulties sometimes are experienced in heat treating steelarticles in fused salt baths because the salt baths usually cause moreor less decarburization of the steel surface. Heretofore thisdecarburizing effect has been overcome by introducing a carburizingmaterial, for example, alkali metal cyanide or other cyanogen compound,into the fused salt bath. By this method the carburization which occurs,more or less compensates for the decarburizing effect of the bath.However, in commercial work it is practically impossible thus tocontinuously maintain-a bath at a substantially neutral condition; thatis, in such condition that the net effect is neither carburization nordecarburization. If the amount of carburizing material is exactlyproportioned so that the carburizing effect equals the decarburizingeffect, the carburizing power of the bath'soon decreases and the steelthen becomes decarburized. If more cyanide is added, the steel is casecarburized.

This problem is less serious when the articles to be treated are ofrelatively massive proportions and the time of heat treatment isrelatively short. However, this tendency of the salt baths to decarburize steel has heretofore made the salt bath method unsuited forpracticable heat treatment of thin, steel articles, such as steel sheetor small .steel wire. For example, when a thin steel sheet is heattreated in a salt bath which has a decarburizing action, the sheet isvery rapidly and almost completely decarburized throughout its depth,thus producing a sheet of entirely different characteristics than thatdesired. On the other hand, when cyanide is added to the bath used forheat treating a steel sheet, the small amount of carburization whichoccurs often is suflicient to substantially entirely carburize the sheetthroughout. Hence, it has heretofore been impossible to heat treat steelsheet or iron sheet in the fused salt bath without materially changingits carbon content to an undesirable extent. For this reason. steelsheet commonly is heat treated in furnaces, such as muffle furnaces, instacks. This involves handling the sheets to stack and unstack them,

and ordinarily the sheets become coated with oxide scale which oftenmust be removed prior to subsequent use. The; stacked sheets often sticktogether, requiring considerable labor to separate the sheets.

- An object of the present invention isto provide a new and improvedmethod for heat treating metal articles in fused salt bathsqff A furtherobject is to provide a satisfactory method for heat treating iron andsteel sheet in afused salt bath. Another object is to provide animproved heat treating fused salt bath whichis substantially neutral incharacter, that is, which neither carburizes nor decarburizes steelarticles treated therein. A still further object is to provide a methodfor heat treating steel articles in fused salt baths without carburizingor decarburizing the steel. Another object is to provide a method foroperating a fused salt bath containing cyanide or equivalent carburizingingredient, whereby the carburizing action of the bath may be inhibitedor controlled. Still other objects will be apparent from the followingdescription of my invention.

The above objects are attained in accordance with the present inventionby heating metal articles in a fused salt bath, preferably onecontaining cyanide or equivalent material, which bath is maintained inan atmosphere of an oxygen-free non-oxidizing gas which is substantiallychemically inert towards the metal treated. I have discovered that thedecarburization and other disadvantageous effects of salt baths on iron,steel, and other metals is caused by the entrance of small amounts ofoxygen into the bath, for example, oxygen from the air. Accordingly, Imaintain an atmosphere of an oxygen-free, non-oxidizing gas over thesurface of the fused salt bath in such manner as to substantiallycompletely prevent the introduction of air or other oxidizing substancesinto the bath.

In the preferred modification of my invention a small amount of. acarburizing material, such as alkali metal cyanide, for example, isadded to the bath. The reason for this is that small amounts ofoxidizing materials inadvertently may be brought into the bath whenoperated over long periods of time. For example, small amounts of airmay remain occluded or adsorbed on the surface of the metal articles tobe treated and also oxide layers on the surface of the metal articlestend in time to impart an oxidizing character to the bath. I have foundthat alkali metal and alkaline earth metal cyanides and cyanamides aswell as other cyanogen compounds such as polymers of hydrocyanic acid,cyanamide,

dicyandiamide and its polymers act as reducing agents infused salt bathsand quickly reduce any oxidizing materials brought into the bath beforethey have an opportunity to affect the metal articles therein.

I have further discovered that, although ordinarily cyanogen compoundssuch as alkali and alkaline earth metal cyanides in fused salt baths arevery active in causing carburization and nitriding of iron or steel,carburization of the iron or steel does not occur to any appreciableextent and the nitriding efiect is decreased to a marked degree orsubstantially eliminated when the salt bath is protected by anatmosphere of non-oxidizing gas in accordance with the presentinvention. Therefore, I may heat treat iron or steel articles in a fusedsalt bathwhich contains even large amounts of a cyanide or othercyanogen compound and so long as the bath is maintained under anatmosphere of non-oxidizing gas, the iron or steel articles treatedtherein will be substantially neither canburized nor decarburized. Bythis means I am able to rapidly and effectively heat treat iron or steelsheet or similar thin iron or steel articles without substantiallyaltering the carbon content thereof. Hence by means of my process, I-amable to heat treat steel sheet more rapidly than heretofore and therebyobtain a superior product with no loss of stock.

I am aware that it has been proposed heretofore to cover the surface offused salt case hardening baths with a floating layer of charcoal orgraphite, which to some extent prevents contact of the air with thesurface of the bath. However, I have found that such protective layersdo not prevent cyanide baths from carburizing iron or steel, nor do theyprevent decarburization of steel when used on non-.carburizing saltbaths. Further, such floating layers mix with the fused salt to form aviscous scum which sticks to the work taken from the bath and isdiflicult to remove therefrom. This is especially disadvantageous in thetreatment of thin articles such as steel sheet. to operate casehardening baths on the surface of which combustible materials, 'e. g.hydrogen -or carbonaceous substances, are permitted to burn. Suchoperations are not contemplated in my invention, since the products ofcombustion (water, carbon oxides) are harmful in contact with mytreating baths and also such layer of burning material does notefiectively prevent diffusion of air into the bath.

In order to attain the objects of my invention it is also, necessary toavoid the presence of solid oxidizing materials in the salt bath, forexample,

strongly oxidizing salts such as nitrates, chromates, and the like. Ingeneral I prefer to avoid the introduction of any oxygenorsulfur-containing salts into the fused salt bath in any proportionswhich will be Jikely to cause oxidation eflfects. However, alkali metalcarbonates may be used as the bath constituents. Accordingly, I preferto use only alkali metal and alkaline earth metal halides, carbonates,and cyanides as the principal bath constituents.

Also, it has ben proposed heretofore Another disadvantage is that thealkaline earth metal salts tend to carburize steel, even under thenon-oxidizing gas atmosphere when the bath is first made up, probablybecause of a reaction such as follows:

The carbon thus formed will tend to carburize iron or steel present whenthe reaction occurs. However, if the bath is heated at the operatingtemperature from one to several hours before use, the bath then willreach a state of equilibrium and thereafter substantially will notcarburize steel treated therein, so long as the bath is properlyprotected by the non-oxidizing gas atmosphere, in accordance with myinvention. In operating such bath to carry out my herein describednon-carburizing heat treating method, I prefer to add a small amount ofcyanide at the conclusion of each days operation and the next day toheat the bath for at least one hour under the non-oxidizing atmospherebefore resuming the heat treating operations.

In the preferred embodiments of my invention, the fused bathswillcontain one or more of the alkali metal chlorides and carbonates,and I mayadd thereto a cyanogen compound, for example 0.540% by weightof sodium cyanide. The invention is not restricted to any particularproportion of cyanogen compound, so long as the bath is substantiallyfluid at the operating temperature. For example, I may employ a bath ofsubstantially pure sodium cyanide or other fusible cyanide withsatisfactory results. Likewise, I may omit the cyanogen compound,although generally I prefer to have cyanide or other cyanogen compoundin the bath. Specific examples of preferred fused bath compositions forpracticing my invention are as follows.

Bath N0. 1

Sodium cyanide: 15-30% by weight. Remainder, sodium carbonate.

Bath N0. 2

Sodium cyanide: 2-20% by weight. Remainder, equal parts of sodiumcarbonate and potassium carbonate.

Bath No. 3

Sodium cyanide: 2-20% by weight. I Remainder, the mixture:

Per cent by weight Barium chloride 50 Potassium chloride Sodium chloride20 Bath N0. 4

Same as Bath No. 3, but 0.05 to 0.5% by weight of solid hydrocyanic acidpolymer in place of the sodium cyanide.

Various other fused baths may be made for practicing my invention, whichcontain one or more of the following salts: alkali metal halides andcarbonates. alkali metal cyanides and alkaline earth metal halides. Suchbaths may be made withor without the cyanides or other cyanogencompounds. The methods of mixing and fusing the salts, as well as theproportions used to obtain baths having melting points adapted for thedesii'ed operating temperatures, will be known to those experienced inthe use of fused salt baths for heat treating metals and need not bedescribed here. a

Various oxygen-free gases may be used to maintain a non-oxidizingatmosphere above the surface of the bath, for example, hydrogen,nitrogen, gaseous hydrocarbons; the rare gases such as argon, krypton,and helium; or ammonia. I

because of its tendency to nitride the iron or steel articles when thearticles are withdrawn from the bath, which is sometimesdisadvantageous. However, unless the ammonia comes into direct contactwith the work or is introduced into the bath, it has substantially nonitriding efiect in my process. Neither carbon dioxide nor carbonmonoxide are suitable for use as a non-oxidizing atmosphere in carryingout. my process. Although carbon monoxide ordinarily is considered anon-oxidizing gas, for the purpose of the present invention it must beconsidered as substantially am, oxidizing material because it reacts atthe bath temperature to form carbon dioxide, and should not be presentin the non-oxidizing gas to any material extent. It is also preferableto use substantially dry gases, since an exces- .sive amount of watervapor will have an undesirable oxidizing effect. I may use either a puregas or various mixtures of non-oxidizing ses in practicing my invention.

The efficacy of my process is shown by comparative experiments, in whichsteel samples were treated in a cyanide bath in contact with air and incontact with a non oxidizinggas. Samples of cold rolled steel bars inchsquare were heated for 30 minutes in a fused bath of substantially puresodium cyanide (analysis, 97.2% NaCN) at a temperature of about 800 C.In the first run,

the cyanide was melted in an air-tight vessel under an atmosphere ofnitrogen of 99.8% purity and the bath was maintained under the nitrogenatmosphere while the steel bars were treated therein. In the second run,the cyanide bath was exposed to the air while the steel was beingtreated therein. The treated samples were water quenched, sectioned,polished, etched with the standard Nital" etching fluid (mixture of 5%nitric acid in alcohol) and examined to determine the extent of casehardening. The samples thus treated in the cyanide bath exposed to theair exhibited .a definite case, clearly visible to the unaided eye. Thecase depth in these samples, measured under the microscope at 100diameters, was found to be about 0.005 inch. On the other hand, thesamples treated under the nitrogen atmosphere had no case visible to theunaided eye. Under the microscope at 100 diameters magnification, thesesamples showed a very slight case, too small to be accurately measuredunder the microscope at 100 diameters, definitely less than 0.001 inch.

Methods for maintaining the atmosphere of non-oxidizing gas overthe heattreating baths will be obvious to those who are acquainted with the artof heat treating metals and need not be described in detail in thisspecification. For example, in heat treating metals it is commonpractice to use a muffle furnace filled with nonoxidizing gas suchashydrocarbon gas or hydrogen and by constantly passing gas into thefurnace so that it flows out through the aperture through which thearticles to be treated are inserted or removed, the entrance of air intothe furnace is prevented. I may apply the same method of operation tomaintain a non-oxidizing atmosphere over the salt bath in practicing myinvention. For example, the fusedbath is maintained in a suitable vesselsimilar to those now in use but which is covered with a suitable tightlyfitting cover or hood provided with a suitable inlet for thenon-oxidizing gas to be used. One or more openings are provided in thecover for the introduction and removal of work to be treated, suchopening or openings being as small as can be conveniently arranged. Solong as these openings are open for the passage offwork, thenon-oxidizing gas is fed into the cover of, the vessel, so that acontinuous stream of non-oxidizing gas issues from those openings, thuspreventing the entrance of air. If desired, such openings may be closedtightly during the heat treating operation.

During idle periods, so longas the heat treating bath is in a fusedcondition, I prefer tomaintain' the non-oxidizing atmosphere over thebath surface, in order to prevent absorption of oxygen or otheroxidizing gases in the bath. When a fused salt bath is freshly made upor if it has for any reason absorbed oxygen or other oxidizing gas, itmay be placed in a non-oxidizing condition by maintaining it in a fusedcondition for several hours, e. g., one or more hours, while passing thenon-oxidizing gas over or through the melt. Alternatively I may alsodeoxidize such bath by introducing a cyanogen compound such as alkalimetal cyanide or other suitable reducing a ent.

' As an example of a specific mode of practicing my invention, I mayheat treat a sheet steel strip by continuously passing the strip througha fused salt bath at such rate that the desired time of heating at thetemperature of the fused salt is attained. To accomplish this inaccordance with my invention, the fused bath is maintained in a vesselwhichis closed by a tightly fitting cover having narrow slots arrangedat opposite sides, which slots are just large enough for the steel sheetstrip to pass through. Rollers may be situated adjacent to these slots,preferably on the outside of the cover, to facilitate the movement ofthe sheet. One or more rollers or other suitable guiding devices arealso arranged within the heat treating vessel in order to guide thesheet below the surface of the fused salt bath-and thence upward and outthrough the exit slot. A stream of a suitable non-oxidizing gas, such ashydrogen or nitrogen, isfed beneath the cover of the vessel at a ratesufficient to prevent entrance or diffusion of air through the openingsthrough which the sheet is passing. Preferably, the distance between thesurface of the fused bath and the point at which the treated sheetemerges into the open air is so arranged that the sheet will be cooledto a temperature below which the steel becomes stained by oxidation inair, e. g., 100-200 C. or lower, before the sheet emerges into the air.I have found that the fused salt which adheres to the work leaving thebath rapidly solidifies to form a brittle layer of solidified has beencooled to a temperature below that at which rapid oxidation occurs," Ican effectually prevent'the undesirable spotty oxidation of the sheet.The adhering solidified salt layer can readily be removed from thecooled. sheet by known means, for example, by mechanical removal or bywashing with water. In' the above described continuous treatment ofsteel strip, the fused bath may be, for example, one of the fourspecific baths described above, e. g. Bath No. l, which consists of amixture of sodium carbonate and sodium cyanide. The 'bath temperaturemay be maintained at a temperature between the melting point of the bathand 900 C. or even higher, as desired, the temperature utilizeddepending upon the nature of the steel and the effect desired. The steelstrip may be passed through the bath at a rate adapted to maintain thesheet in contact with the bath for a period of time from a few secondsto several minutes, depending upon the thickness of the sheet and thedegree of recrystallization desired. The resulting product hassubstantially the same carbon content as the untreated sheet.

The continuous method of heat treating sheet steel strips described inthe foregoing paragraph may be better understood by reference to theaccompanying drawing showing a vertical sectional view of one form of anapparatus which may be used to practice the method. In the drawing 1 isa heat treating vessel containing a fused salt bath 2. The upper portionof the side walls of vessel I are flanged outwardly so as to form flange3. and the lower portions of the side walls of cover 4 are flangedoutwardly so as to form flange 5. Cover 6 is secured to the top of theheat treating vessel l by means of flanges 3 and 5 which are securedtogether by means of bolts 6 so as to form a relatively gas tight fitbetween the vessel and the cover. Cover 4 is provided with ahorizontally extending cooler section I and non-oxidizing gas isdelivered under cover 4 by way of conduit 8 which is provided with valveIn. Non-oxidizing gas may escape from cover 4 by way of conduit 9 whichis provided with valve I I.

Metal-strip to be treated passes into the apparatus by way of slot l2and out of the apparatus by way of slot I3. Slots l2 and I3 should belarge enough to permit passage of the metal strip therethrough, but notlarge enough to permit entrance of any substantial amount of air intothe apparatus by way of these slots. If desired, each side of the metalstrip as it enters or emerges from the apparatus may be scraped byresilient contacting members (not shown in the drawing) secured to theadjacent walls of the cover in order to insure against entrance of anysubstantial amount of air into the apparatus by way of slots l2 and I3..However, the introduction of non-oxidizing gas into the cover in suchamounts and at such a rate that streams of the gas are caused to escapefrom the apparatus by way of slots l2 and I3 generally provides adequateassurance against the entry of air into the apps.

ratus.

, The passage of 'the metal strip Ill through the apparatus isfacilitated by means of rollers I5, IS, IT, I8, I9 and 20. Elements 2|represent wipers which serve to remove excess fused salt compositionwhich may adhere to the steel strip as it emerges from bath 2. However,such wipers are not necessary, since excess adhering fused saltcomposition may be easily removed from the treated metal stripmechanically or by washing after the strip has emerged from theapparatus.

' It will be noted that the cooler section 1 is shown as being connectedto cover 4 by a. flange connection 22. Attachment of the cooler to theVessel-l is provided with a cover 4 cover may be made in any otherconvenient manner. The length of cooler i will depend upon the amount ofcooling required to reduce the temperature of the steel strip to about200 C. or lower, e. g., to a temperature below that at which steelbecomes rapidly stained by oxidation in air. If necessary, cooler I maybe cooled externally, e. g. by means of cold air or cold water. Itshould be noted that by adjusting the rate of flow of non-oxidizing gasthrough conduits 8 and 9 by means of valves It and! 5, respectively, itwill be possible to prevent any substantial entrance of -air into theapparatus. Thus by introducing gas into the cover by way of conduit 8 ata greater rate than that at which gas is removed by way of conduit 9, aportion of the non-oxidizing gas will be forced out of the apparatus byway of slots I 2 and 13 so that the flow of gas through slots I2 and E3or any other opening in the apparatus will be from the inside out ratherthan in the reverse direction. Cover 4 will normally be provided withdoor or gate arrangements, or the like, not shown in the drawing, forfacilitating the introduction of salt composition into vessel l and forother obvious purposes. Various modifications of the apparatusillustrated in the drawing may be used.

In addition to non-carburizing heat treating operations, my inventionmay be practiced to effectively control the case hardening activity ofcase hardening baths in which cyanide or other cyanogen compound is usedas the case hardening ingredient. That is, by varying the proportion ofoxidizing gas in the atmosphere above the case hardening bath, the casehardening activity of the bath may be varied accordingly. For example, acyanide case hardening bath may be operated under an atmosphere ofnitrogen or other non-oxidizing gas and a suitable proportion of oxygenor other oxidizing gas may be mixed with the nitrogen. To decrease theactivity of the bath, the atmosphere above the bath may for exampleconsist of a nitrogen-oxygen mixture containing an oxygen concentrationbelowthat of the air, e. g., 1-5% by volume of oxygen. To increase theactivity of the bath, the covering atmosphere may, for example, consistof air to which oxygen has been added. Thismethod of control isgenerally applicable to case hardening baths containing cyanogencompounds and is not restricted to baths activated with cyanide.

Although I have described my invention as primarily adapted for the heattreatment of iron and steel articles and more particularly for thetreatment of steel sheet, my process is also suitable for heat treatingarticles of other metals, for example, copper and its alloys such asbrass or bronze; nickel and its alloys; and various other metals andalloys. The non-oxidizing character of my improved heat treating processis a distinct advantage in treating such readily oxidized materials ascopper, brass, and the like. By my process such readily oxidized metalsmay be heat treated with substantially no surface oxidation, therebyeliminating after-treatments to remove scale or color.

My invention is not restricted to any particular temperature of heat"treatment but is generally applicable to the various heat treatingtemperatures commonly utilized for fused salt bath metal heat treatingoperations, e. g., 450-900 C.

I claim:

1. A process for heat treating a-metal article which comprises immersingsaid article in a substantially non-oxidizing fused salt bath containinga cyanogen compound while maintaining a substantially oxygen-freeatmosphere of a nonoxidizing gas above the surface of said bath.

2. A process for heat treating a metal article which comprises immersingsaid article in a substantially non-oxidizing fused salt bath containinga cyanogen compound while maintaining a substantially oxygen-freeatmosphere of a nonoxidizing gas above the surface of said bath, and

thereafter removing said article from said bath into said oxygen-freeatmosphere and cooling said article in said atmosphere.

3. A process for heat treating ferrous metal articles which comprisesimmersing said articles in a substantially non-oxidizing fused salt bathpreponderantly composed of at least one salt selected from the alkalimetal and alkaline earth metal halides, alkali metal carbonates andcyanides, which bath contains a cyanogen compound, While maintaining asubstantially oxygen-free atmosphere of a non-oxidizing gas above thesurface of said bath.

4. A process for heat treating ferrous meal articles which comprisesimmersing said articles in a substantially non-oxidizing fused salt bathpreponderantly composed of at least one salt selected from the alkalimetal and alkaline earth metal halides, alkali metal carbonates andcyanides, which bath contains a cyanogen compound, while maintaining asubstantially oxygenfree atmosphere of a non-oxidizing gas above thesurface of said bath, and thereafter removing said articles from saidbath into said oxygenfree atmosphere and cooling said articles in saidatmosphere.

5. A process for hea ttreating ferrous metal articles which comprisesimmersing said articles in a substantially non-oxidizing fused salt bathcontaining a cyanogen compound, while maintaining a substantiallyoxygen-free atmosphere of a non-oxidizing gas above the surface of saidbath.

6. A process for heat treating ferrous metal articles which comprisesimmersing said articles in a substantially non-oxidizing fused salt bathcontaining an alkali metal cyanide, while maintaining a substantiallyoxygen-free atmosphere of a non-oxidizing gas above the surface of saidbath.

7. A process for heat treating ferrous metal articles which comprisesimmersing said articles in a substantially non-oxidizing fused salt bathpreponderantly composed of at least one salt selected from the alkalimetal and alkaline earth metal halides, alkali metal carbonates andcyanides, containing about 0.5 to 40% by weight of at least one of saidcyanides, while maintaining a substantially oxygen-free atmosphere of anonoxidizing gas above the surface of said bath.

8. A process for heat treating steel articles which comprises immersingsaid articles in a substantially non-oxidizing fused salt bathpreponderantly composed of at least one salt selected from the alkalimetal and alkaline earth metalhalides, alkali metal carbonates andcyanides, which contains an alkali metal cyanide, while maintaining asubstantially oxygen-free atmosphere of a non-oxidizing gas above thesurface of said bath.

9. A process for heat treating steel articles which comprises immersingsaid articles in a substantially non-oxidizing fused salt bathpreponderantly composed of at least one sailt selected from the alkalimetal and alkaline earth metal halides, alkali metal carbonates andcyanides, containing about 0.5 to 40% by weight of sodium cyanide, whilemaintaining a substantially oxygen-free atmosphere of nitrogen above thesurface of said bath.

10. A process for heat treating steel sheet which comprises immersingsaid sheet in a substantially non-oxidizing fused salt bath containing acyanogen compound, while maintaining a substantially oxygen-freeatmosphere of a nonoxidizing gas above the surface of said bath.

11. A process for heat treating steel sheet which comprises immersingsaid sheet in a substantially-non-oxidizing fused salt bathpreponderantly composed of at least one salt selected from the alkalimetal and alkaline earth metal halides, alkali metal carbonates andcyanides, containing about 0.5 to 40% by weight of an alkali metalcyanide, while maintaining a substantially oxygen-free atmosphere ofnitrogen above the surface of said bath.

12. A process for heat treating steel sheet which comprises immersingsaid sheet in a substantially non-oxidizing fused salt bathpreponderantly composed of at least one salt selected from the alkalimetal and alkaline earth metal halides, alkali metal carbonates andcyanides, containing about 0.5 to 40% by weight of an alkali metalcyanide, while maintaining a substantially oxygen-free atmosphere ofhydrogen above the surface of said bath. 7

13. The process for heat treating sheet steel strip which comprisescontinuously passing said strip through a substantially non-oxidizingfused salt bath containing an alkali metal cyanide, while maintaining anatmosphere of substantially non-oxidizing gas over the surface of saidbath.

14. The process for heat treating sheet steel strip which comprisescontinuously passing said strip through a substantially non-oxidizingfused salt bath containing an alkali metal cyanide, while maintaining anatmosphere of substantially non-oxidizing gas over the surface of saidbath and maintaining the portion of said strip emerging from the bath insaid non-oxidizing gas until the strip has cooled to a temperature belowabout 200 C.

DONALD A. HOLT.

CERTIFICATE OF CORRECTION. Patent No. 2,251,009. February 11, 19in,

DONALD A. HOLT.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 5,first column, line 25, claim )4, for the word "meal" read --metal--;line 56, claim 5, for "hea ttreating" read heat treating"; same page,second column, line 12, claim 9, for "sailt" read --salt-; and that thesaid Letters Patent should be read with this correction therein that thesame may conform to the record of the case in the Patent Office.

Signed and sealed this 25th day of March, A. D. 19LL1.

Henry Van Arsdale (Seal) Acting Commissioner of Patents.

